Stimulated Raman scattering (SRS) is an important technique for shifting the optical outputs of available laser systems to shorter or longer wavelengths. In this way, coherent radiation is produced in regions of the optical spectrum which are not otherwise covered by combinations of available (fixed-wavelength) laser systems and crystalline nonlinear media, in particular, the mid- and far-infrared (IR) and the ultraviolet (UV) and vacuum-ultraviolet (VUV) regions of the electromagnetic spectrum. The spontaneous Raman scattering effect was discovered by Raman in 1923, who shone focussed filtered sunlight into different materials. Raman found, by analyzing the light emitted by the material, colors of light different than those he had applied to the medium. The color difference represented a difference in photon energies that Raman subsequently found to be characteristic of the particular material being irradiated. The difference in photon energies turned out to be equal to energies of transitions, either electronic, vibrational, or rotational, in the material. Quantum mechanically, the incident light mixes with the zero-point fluctuations (in a manner similar to the buildup of superfluorescence); the atom or molecule simultaneously absorbs an incident photon and emits a second photon at a (typically) longer wavelength (less energetic). The difference in photon energies causes a change in the internal energy of the atom or molecule, and the internal electronic distribution is changed.
The most common modern arrangement for the demonstration of SRS involves a pump laser, providing intense coherent radiation at a given wavelength, and a single vessel containing a Raman-active medium (which may be solid, liquid, or gasseous). A schematic diagram of such an arrangement is shown in FIG. 1. Typically, the output of a pulsed Q-switched laser is focussed to a high intensity (typically several GW/cm2) into a pressurized cell 100 containing a Raman-active medium 105 (the Raman cell) by suitable lenses. Under these conditions, the Raman process evolves from a spontaneous single-photon process to a stimulated scattering (SRS) process involving many photons. An appreciable fraction of the pump laser energy 101 may be converted by SRS into a “comb” of sidebands 102, equally spaced by the Raman transition frequency, above and below the pump field frequency 121. Sidebands with frequencies less than the pump frequency 121 are known as the Stokes sidebands 122, and those with frequencies higher than the pump are the anti-Stokes sidebands 123. Raman shifting was the first nonlinear effect discovered after the invention of the laser in 1960, and as such, there is a great body of research on the subject. Common Raman gasses are molecular hydrogen (H2) and deuterium (D2). The number of these sidebands and their relative intensities depend upon the particular parameters of the apparatus. The most common variables in a gasseous Raman scattering experiment are the gas pressure 105 and the pump laser intensity, which may be varied by changing the focussing properties of the pump laser beam 101. Many examples of results using this type of apparatus exist in the prior art. Raman scattering is also used in the detection and identification of unknown media, as each material is characterized by a unique scattering “fingerprint”.
The limitations of the simple arrangement shown in FIG. 1 (single pump laser beam 101 and single Raman cell 100) are well known, there are fundamental limitations on the fraction of incident pump energy that may be selectively scattered into a particular Stokes or anti-Stokes order. The generated sidebands 102 themselves are characterized by a relatively low degree of mutual coherence as compared to those generated using the techniques of the present Application.
This invention is particularly concerned with the simultaneous application of the pump laser frequency and the first Stokes sideband to a Raman medium—a process known as “Stokes injection” —to increase sideband generation efficiencies. It has recently been found that Stokes injection significantly enhances the SRS process. Previous experiments have noted dramatic improvements in shot-to-shot amplitude stability, spatial mode profiles, and conversion efficiencies. The increase of efficiency relative to the single-cell Raman shifters is most notable in the higher anti-Stokes orders.
More recently still, it also has been shown that the nonlinearity of the Raman medium available for the production of the plurality of sidebands 102 depends sensitively on the linewidths and exact frequency difference of the driving fields. Maximizing the Raman nonlinearity is desirable in order to maximize the extent and energy of the “comb” of generated sidebands. Whenever the Raman sidebands build up from quantum fluctuations, as is the case whenever a single pump laser and Raman cell are employed, the frequency separation between said sidebands is exactly equal to the frequency of the Raman transition. Equivalently, if a weak Stokes signal is injected into the Raman medium to enhance the scattering process, maximum gain is experienced when the Stokes signal frequency drives the Raman medium substantially on resonance. For this reason, to date, most experiments that have demonstrated Stokes injection have done so under conditions where the frequencies of the pump and first Stokes sidebands are adjusted so that their frequency difference is exactly equal to a two-photon vibrational transition resonance in the Raman medium.
Conversely, when Stokes signals of higher intensity (several GW/cm2) are injected, the nonlinearity of the Raman medium maximizes when the driving lasers are detuned slightly from exact two-photon resonance. This curiosity arises because of the nature of Doppler broadening and the adiabatic excitation of a substantial fraction of the molecular population into a specific eigenstate. The driving of a Raman-active medium is shown schematically in FIG. 2 for off-resonance (non-zero δν) driving fields 210, 211. The two-photon frequency detuning δν 220 is defined as νR−(νP−νB), where νR is the Raman transition frequency (νR=ν2−ν1) and νP and νB are the frequencies of the pump 210 and first-Stokes sideband 211, respectively. A wide comb of sidebands may be generated when the frequency detuning between the pump laser 210 and the electronic excited states 202 is large.
The numerical simulations shown in FIG. 3 illustrate the shift in optimum two-photon detuning δν 220 away from exact resonance as a function of the applied pump 210 and Stokes-field 211 intensities IP and IB in H2 gas at a pressure of 2.5 atmospheres at 298K. The Raman nonlinearity—specifically, the square of the Doppler-averaged Raman transition coherence at the pulse peak—is plotted against the two-photon frequency detuning of the driving lasers. Zero detuning 311 corresponds to exact two-photon resonance with the pertinent Raman transition. Parameters for these plots are as follows: applied lasers with 10-ns-pulsewidths, and intensities Ip=IB=(a) 300 MW/cm2, (b) 900 MW/cm2, (c) 1500 MW/cm2, and (d) 2750 MW/cm2. It is clear from these data that the two-photon detunings that maximize the Raman nonlinearity depend upon the applied field intensities. In paricular, at low field intensities, the largest nonlinearities 310 occur exactly on-resonance 311, but when high-intensity fields are applied, maximum nonlinearities 321, 322 are instead obtained when the driving laser frequencies are tuned so that the two photon detuning is several hundred MHz, approximately one Doppler width, from the center of the Raman resonance 311. The frequency separation of the two peaks 321, 322 depends upon the laser field intensity, while the magnitude of the nonlinearity depends upon the Doppler width and decoherence mechanisms present in the Raman medium. Note that at high field intensities, the Raman nonlinearity reaches a minimum 320 near Raman resonance 311. Experimental verification of these trends may be found in A. V. Sokolov et al., “Raman Generation by Phased and Antiphased Molecular States”, Phys. Rev. Lett. 85, 562 (2000). The experimental apparatus of Sokolov employed two separate, powerful, costly single-mode pulsed laser systems, the precise frequency difference of which could be easily adjusted.
The numerical simulations of FIG. 3 are known as ‘time-only’ simulations: they simulate the macroscopic temporal behavior of a distribution of irradiated molecules at a given point in space. In reality, the Raman interaction occurs in a region of space defined by the interacting beams, i.e., across a certain length of the Raman medium; thus, the sideband ‘comb’ is generated, and continues to interact, over a distribution of space. Estimation of the generation efficiency of any particular sideband in the ‘comb’ should therefore consider the propagation of the electromagnetic fields over this region. Simulations incorporating space as well as time have corroborated the experimental observations of increased sideband generation efficiency using off-resonance drive fields. Further, these simulations have confirmed that said generation efficiency increase is partially due to the increased adiabaticity of the off-resonant molecule-field interaction. One measure of the adiabaticity of the Raman-type interaction considered in this Application is the fraction of energy which remains in the medium after the fields have traversed said medium. Energy in a Raman interaction flows from the applied electromagnetic fields to both the comb of generated sidebands and the material itself. The more adiabatic the interaction, the larger the fraction of energy which may propagate through said medium and which is therefore available for sideband generation. For a given amount of material excitation, increased adiabaticity results in increased sideband generation.
It is worth noting that high-intensity fields tuned to exact Raman resonance 311 excite only a small fraction of the available Raman nonlinearity 320 (low material excitation), as shown in FIG. 3. This type of interaction represents the usual manner by which previous ‘enhanced’ Raman shifters were operated. Similarly reduced material excitation 330 is observed when large two-photon frequency detunings are employed. Additionally, the interaction of the molecules with said intense, resonant fields is strongly non-adiabatic, causing a significant reduction in the amount of energy available for sideband generation. These conditions, which are characteristic of Raman-shifters of the prior-art, have significantly reduced the sideband generation efficiencies of said prior-art shifters.